This invention relates generally to expanded biodegradable starch-containing products with improved flexibility, pliability or resiliency and other properties appropriate for use as packaging materials, particularly for extruded sheets, molded trays and other packaging containers. The invention includes improved methods for manufacture of expanded starch products and starch-containing extruder feed compositions.
Biodegradable materials are presently in high demand for applications in packaging materials. Commonly used polystyrene (xe2x80x9cStyrofoamxe2x80x9d (Trademark)), polypropylene, polyethylene, and other non-biodegradable plastic-containing packaging materials are considered detrimental to the environment. The use of such non-biodegradable materials will decrease as government restrictions discourage their use in packaging applications. Biodegradable materials that are flexible, pliable and non-brittle are needed in a variety of packaging applications, particularly for the manufacture of shaped biodegradable containers for food packaging.
For such applications, the biodegradable material must have mechanical properties that allow it to be formed into and hold the desired container shape, being resistant to collapse, tearing or breaking. In addition, biodegradable materials with resiliency, compressibility and bulk density comparable to xe2x80x9cStyrofoamxe2x80x9d (Trademark) xe2x80x9cpeanutsxe2x80x9d are needed in loose-fill packaging applications. Further, biodegradable sheet materials having properties comparable to polypropylene and polyethylene materials are needed, for example, in laminating packaging materials.
Starch is an abundant, inexpensive biodegradable polymer. A variety of biodegradable starch-based materials have been proposed for use in packaging applications. Conventional extrusion of starch produces expanded products that are brittle, sensitive to water and unsuitable for preparation of packaging materials. Attempts to prepare starch-based products with flexibility, pliability or resiliency and other mechanical properties acceptable for various biodegradable packaging applications have generally focused on chemical or physico-chemical modification of starch, the use of expensive high amylose starch or mixing starch with synthetic polymers to achieve the desired properties while retaining biodegradability. A number of references relate to extrusion, and injection molding of starch-containing compositions.
U.S. Pat. Nos. 4,133,784, 4,337,181, 4,454,268, 5,322,866, 5,362,778, and 5,384,170 relate to starch-based films that are made by extrusion of destructurized or gelatinized starch combined with synthetic polymeric materials. U.S. Pat. No. 5,322,866 specifically concerns a method of manufacture of biodegradable starch-containing blown films that includes a step of extrusion of a mixture of raw unprocessed starch, copolymers including polyvinyl alcohol, a nucleating agent and a plasticizer. The process is said to avoid the need for pre-processing of the starch. U.S. Pat. No. 5,409,973 reports biodegradable compositions made by extrusion from destructurized starch and an ethylene-vinyl acetate copolymer.
U.S. Pat. No. 5,087,650 relates to injection-molding of mixtures of graft polymers and starch to produce partially biodegradable products with acceptable elasticity and water stability. U.S. Pat. No. 5,258,430 relates to the production of biodegradable articles from destructurized starch and chemically-modified polymers, including chemically-modified polyvinyl alcohol. The articles made are said to have improved biodegradability, but to retain the mechanical properties of articles made from the polymer alone. U.S. Pat. No. 5,292,782 relates to extruded or molded biodegradable articles prepared from mixtures of starch, a thermoplastic polymer and certain plasticizers. U.S. Pat. No. 5,403,875 relates to blends of starch with acrylic polymers which are melt-processed into thermoplastic articles. U.S. Pat. No. 5,393,804 relates to biodegradable compositions made from starch melts containing converted starch and plasticizers including polyvinyl alcohol. U.S. Pat. No. 5,095,054 concerns methods of manufacturing shaped articles from a mixture of destructurized starch and a polymer.
U.S. Pat. No. 4,125,495 relates to a process for manufacture of meat trays from biodegradable starch compositions. Starch granules are chemically modified, for example with a silicone reagent, blended with polymer or copolymer and shaped to form a biodegradable shallow tray.
U.S. Pat. No. 4,673,438 relates to extrusion and injection molding of starch for the manufacture of capsules. U.S. Pat. No. 5,427,614 also relates to a method of injection molding in which a non-modified starch is combined with a lubricant, texturizing agent and a melt-flow accelerator.
U.S. Pat. Nos. 4,863,655, 5,035,930 and 5,043,196 report expanded starch materials in which the starch has at least 45% by weight amylose (high amylose materials). Expanded high amylose starch compositions have also been modified by reaction with oxiranes (e.g. ethylene oxide) and include the commercially available xe2x80x9cECO-FOAMxe2x80x9d (Trademark) in which the starch is modified by reaction with propylene oxide. Expanded packaging materials made from high amylose starch are too expensive for practical commercial use. U.S. Pat. No. 5,314,754 of Knight (May 24, 1994) reports the production of shaped articles from high amylose starch.
EP published application EP 712883 (published May 22, 1996) relates to biodegradable, structured shaped products with good flexibility made by extruding starch having defined large particle size (e.g., 400 to 1500 microns). The application, however, only exemplifies the use of high amylose starch and chemically-modified high amylose starch.
U.S. Pat. No. 5,512,090 refers to an extrusion process for manufacture of resilient, low density biodegradable packaging materials, including loose-fill materials, by extrusion of starch mixtures comprising polyvinyl alcohol (PVA) and other ingredients. The patent refers to a minimum amount of about 5% by weight of PVA. U.S. Pat. No. 5,186,990 reports a lightweight biodegradable packaging material produced by extrusion of corn grit mixed with a binding agent (guar gum) and water. Corn grit is said to contain among other components starch (76-80%), water (12.5-14%), protein (6.5-8%) and fat (0.5-1%).
U.S. Pat. No. 5,208,267 reports biodegradable, compressible and resilient starch-based packaging fillers with high volumes and low weights. The products are formed by extrusion of a blend of non-modified starch with polyalkylene glycol or certain derivatives thereof and a bubble-nucleating agent, such as silicon dioxide. U.S. Pat. No. 5,252,271 of Hyrum (Oct. 12, 1993) reports a biodegradable closed cell light weight packaging material formed by extrusion of a modified starch. Non-modified starch is reacted in an extruder with certain mild acids in the presence of water and a carbonate compound to generate CO2. Resiliency of the product is said to be 60% to 85%, with density less than 0.032 g/cm3.
U.S. Pat. No. 3,137,592 relates to gelatinized starch products useful for coating applications produced by intense mechanical working of starch/plasticizer mixtures in an extruder. Related coating mixtures are reported in U.S. Pat. No. 5,032,337 to be manufactured by extrusion of mixture of starch and polyvinyl alcohol. Application of thermomechanical treatment in an extruder modifies the solubility properties of the resultant mixture which can then used as a binding agent for coating paper.
While significant progress has been made toward non-brittle, starch-based biodegradable packaging materials, there nevertheless remains a significant need for lowering production costs and improving the physical and mechanical properties of extruded starch materials.
The present invention provides methods for producing biodegradable expanded starch products with improved properties for packaging applications. These methods comprise the steps of extruding a starch-containing mixture through an expansion die, followed by compressing the extrudate, for example, by subjecting the extrudate to pressure generally perpendicular to the direction of extrusion. Compression can be applied, for example, by use of a rolling device. The extrusion can be performed using conventional methods and apparatus. Die size, die shape and/or type of compression used, e.g., roller configuration, are selected to produce a shaped, compressed or rolled extrudate. The compressed or rolled exudate is optionally subjected to further shaping, molding and/or cutting to obtain a desired final shaped article to suit the planned application.
It has been found that compressing, or compression combined with stretching of the starch-based extrudate significantly improves the flexibility, pliability, mechanical strength and dimensional stability of extruded, pressure molded packaging containers. The preferred method for compression of the extrudate is by rolling. It has also been found that compression or rolling improves the resiliency of extruded, low density expanded starch articles, such as those useful for loose fill applications. Significant decreases in the brittleness of extruded starch products are achieved by compressing, rolling, compressing and stretching or rolling and stretching of the material after extrusion. Application of the methods of this invention allows production of expanded starch packaging materials having mechanical properties suitable for packaging application using significantly lower levels of expensive polymeric plasticizers, e.g., polyols such as polyvinyl alcohol.
In the method of this invention, pressure can be applied to the emerging extrudate in a direction generally perpendicular to the direction of extrusion. Pressure is preferably applied substantially perpendicular to the direction of extrusion. The pressure applied is preferably sufficient to substantially compress the cells in the extrudate. Surprisingly, it is believed that substantially compressing, or preferably crushing, the cells of the expanded product significantly improves the flexibility, pliability or resiliency of the product and significantly reduces brittleness. The preferred method of applying pressure to the extrudate is rolling the extrudate between opposed rollers. The amount of pressure applied may be varied dependent upon the desired final properties of the material. Rolling is preferably done while the extrudate is still hot, most preferably immediately upon exit of the extrudate from the extruder die.
In another aspect, the present invention provides a method of producing improved expanded starch products comprising the steps of extruding a starch-containing mixture through an expansion die, followed by stretching the extrudate, for example, by pulling the extrudate in the direction of extrusion. Stretching can be applied, for example, by pulling the extrudate strand, substantially in the direction of extrusion, faster than the rate of extrusion. Extrusion can be performed using conventional methods and apparatus. Die size, die shape and/or stretching rate applied are adjusted to provide desired final properties in the extruded product. The stretched extrudate is optionally subjected to further shaping, molding and/or cutting to obtain a desired final shaped article to suit the planned application. Stretching of the starch-based extrudate significantly improves the final properties of shaped extruded articles, including pressure molded packaging containers.
In one embodiment of the method, the extrudate is compresses without substantial stretching. The preferred way to achieve compression without substantial stretching is to employ rollers in which roller speed is substantially matched to the rate of extrusion, so that minimal lateral pressure is applied to stretch the extrudate. In another embodiment of the method, the extrudate is stretched, for example by pulling the extrudate strand as it exits the die at rate faster than the extrusion rate. Preferred stretch rates range from about 1.1 to about 1.5.
In yet another embodiment, the extrudate is compressed and stretched. The extrudate can be stretched before, after or at the same it is being compressed. Preferably stretching is applied at the same time as compression. The preferred way to compress and stretch the extrudate is to employ rollers in which the roller speed is set faster than the rate of extrusion. In this case, the extrudate is both rolled and stretched. Roller speeds for stretching are typically set to be less than or equal to 50% faster than the extrusion rate. Preferred roller speeds for stretching are between about 10% to about 20% faster than the extrusion rate. However, the desired amount of stretching is dependent upon the desired final properties of the product as used herein, a roller speed of 10%, 20%, or 50% faster than the extrusion rate means a 1.1, 1.2 or a 1.5 stretch rate of the extrudate, respectively.
The compressed, stretched, compressed and stretched expanded starch product is biodegradable and has low brittleness and other properties making it attractive for use in packaging applications.
In one embodiment, this invention provides improved starch sheets for cushioning and packaging applications. Sheets are preferably prepared by rolling, or rolling and stretching the extrudate, using a flat roller. Rolled expanded starch sheets are generally useful in packaging and wrapping applications and in the manufacture of laminated materials for packaging applications, such as for manufacture of mailing envelopes and related containers. Single-ply, two-ply or multiple ply sheets can be formed by the method of this invention.
In another embodiment, this invention provides improved molded starch articles, particularly articles that are pressure molded from extruded compressed, rolled or rolled and stretched starch sheets. Molded articles can have various sizes and shapes including shallow and deep trays, cups, tubs, tubes and other containers. Molded articles can also be single or multiple compartment trays or containers and can also be shaped with internal or external ridges, lips, or rims. The starch extrudate is compressed, rolled, or rolled and stretched prior to pressure molding. Pressure molding can employ conventional processes and equipment.
In yet another embodiment, this invention provides improved expanded starch materials having high resiliency, suitable compressibility, low density, low dusting, and low brittleness for use in loose fill packaging applications. These improved starch materials are produced by methods described herein by extrusion, combined with a step of compressing or substantially crushing the cells in the extrudate, preferably by rolling the hot extrudate. Loose-fill can be produced in a variety of shapes by choice of suitable combinations of die and roller shapes. This invention provides a loose fill expanded starch product with resiliency higher than 50% useful for packaging applications. Loose-fill products of this invention have compressibility, flexibility properties comparable to xe2x80x9cStyrofoamxe2x80x9d loose fill.
The methods of this invention can be employed essentially with any starch composition suitable for extrusion. In particular and preferably, the method can be used with starch compositions in which the major ingredient is raw non-modified starch. The method of this invention allows the use of significantly lower levels of plasticizers in the starch extruder feed than previously known to achieve desired mechanical properties in the final expanded or molded starch products.
The invention also provides extruder feed mixtures containing raw, non-modified starch for production of expanded starch products which contain lower levels of plasticizers, particularly lower levels of polymeric plasticizers, such as PVA and related polyols. Extruder feed mixtures of this invention may comprise non-modified starch. To achieve lower production costs, extruder feed mixtures preferably contain less than 5% (by weight) PVA or related polyhydric plasticizer. More preferred extruder feed mixtures contain less than or equal to about 2.5% PVA or related polyhydric plasticizer. Most preferred extruder feed mixtures contain no PVA or related polyhydric plasticizer.
It has been found that lighter weight expanded starch articles and expanded starch pressure molded articles with good mechanical strength and dimensional stability can be produced by the process of this invention from extruder feed mixtures that contain talc.
In another aspect, the invention provides a new extrusion device in which a conventional extruder is combined with a pressure device which can apply pressure to an extrudate emerging from an extruder die substantially perpendicular to the direction of extrusion. The pressure applied being sufficient to substantially compress or preferably to crush the cells in the hot extrudate. The amount of pressure applied and the length of application time of pressure depends upon the desired final properties of the product.
Additional aspects and features of the invention will become apparent in the following detailed description of the invention.